I have my own web server but I want to create two subdomains (Eg. domain1.mydomain.com and domain2.mydomain.com) with the same domain name as the main domain (eg. Mydomain.com). How can I achieve this once users access those subdomains? Is it possible?

Let me get this straight, you have a website. And you're wondering how to create subdomains on it, correct? If that's all you're doing then it should be pretty easy depending on your setup. What type of hosting are you using? A lot of hositng providers will give you the option and do all the nasty, behind the scenes stuff for you.Just to make sure, look at https://www.hackthissite.org/forums. Now look at https://forums.hackthissite.org. Same thing, the second subdomain is just forwarded to the forum index.

You might be able to forward it. I don't see why you would need to do this. Try explaining exactly what you want to do, and maybe we can help from there. You can have a domain be forwarded to another domain. For example, gmail.com forwards to google.com/allthatgibberishshit. But once it forwards to the latter domain, it'll never go back to the former one unless users are somehow directed back there.

What I pretend is like "phising" my primary domain name (with the original webserver. Ther server I pretend being hacked) to a fake "domain name" just to distract attackers with a fake domain name pointing to a fake webserver with that subdomain. Same URL different webservers. Thats the main idea. I've found on the web something about PHP, capable to change url. Is it true?